1. Field of the Invention
This invention relates to a high frequency signal amplifying circuit, and a receiver for a broadcast, for example the DAB (Digital Audio Broadcasting) system to which the high frequency signal amplifying circuit is applied.
2. Description of the Related Art
Recently, transmission systems for broadcasting have been changed to digital systems from analog systems such as conventional AM (Amplitude Modulation) system or FM (Frequency Modulation) system in accordance with a progress of digitalization.
In Europe, a digital broadcasting for audio signals has already started, and it is commonly called as the DAB (Digital Audio Broadcasting) system. A full constitutional figure of a DAB receiver for the DAB system is depicted in FIG. 3.
Broadcasting waves received by an antenna 1 are supplied to a front-end section which includes a high frequency signal amplifying circuit 2 as shown in FIG.3. In this front-end section, a received signal is supplied to a mixing circuit 3 after amplified by the high frequency signal amplifying circuit 2. This amplified signal from the high frequency signal amplifying circuit 2 is then frequency-converted at this mixing circuit 3 into an intermediate frequency signal by a local oscillation signal supplied from a local oscillation circuit 4.
Thus obtained intermediate frequency signal from the mixing circuit 3 is then amplified at an intermediate frequency signal amplifying circuit 5 by about 60 dB, and then demodulated at a demodulation circuit 6. A demodulated signal from the demodulation circuit 6 is supplied to an A/D (Analog to Digital) converter 7. In this case, a signal level of the demodulated signal from the demodulation circuit 6 is adjusted to be about 1V p--p signal level that is required by the A/D converter 7 as an input level.
A digital signal from the A/D converter 7 is supplied to a base-band processing circuit 8, and after being digitally demodulated at this base-band processing circuit 8, a digitally demodulated signal from the base-band processing circuit 8 is then supplied to a D/A (Digital to Analog) converter 9. At this D/A converter 9, the digitally demodulated signal is converted into an analog audio signal, and finally derived from a speaker 10 as an audio sound.
In the DAB receiver as depicted in FIG.3, as the received signal level is increased, an AGC (Automatic Gain Control) circuit 11 connected to the intermediate frequency signal amplifying circuit 5 starts its operation and controls operation of the high frequency signal amplifying circuit 2. As well known, when the level of the received signal supplied to the high frequency signal amplifying circuit 2 and/or the mixing circuit 3 exceeds a predetermined value, the received signal becomes distorted. In order to avoid this kind of distortion, a gain of the high frequency signal amplifying circuit 2 is controlled to reduce the level of the received signal.
As well known, the OFDM (Orthogonal Frequency Division Multiplex) system is employed to a modulation system of the DAB system. A feature of this OFDM system is to employ a multi-carrier system that carries out demodulation while receiving simultaneously plural carriers. For example, in the technical specification called the DAB mode 2, 384 kinds of carriers are transmitted in a transmission bandwidth of 1.536 MHz with the space of 4 kHz. In order to demodulate these plural carriers, a high frequency signal amplifying circuit is required to have a low distortion characteristics such as the IM3&gt;50dBc, wherein the IM3 means the third inter modulation distortion.
Generally speaking, when two signals (F1 and F1+Fd) apart from each other by Fd in frequency are supplied to an amplifier, there causes distortions in an output of the amplifier at frequencies (F1-Fd and F1+Fd+Fd) apart from each other by Fd due to distortion at the amplifier. These distortion signals are called as the IM (Inter Modulation) signals, and level differences between the level of the input signals (F1) and the distortion signals (F1-Fd) are called as the IM3.
Because a broadcasting service is done at high frequency such as 1.4 GHz band in the DAB system, the DAB receiver is required to have a high frequency signal amplifying circuit having high linearity and being equipped with a gain control circuit upon operating at high frequency.
Namely, the high frequency signal amplifying circuit 2 for the DAB receiver requires to be controlled its gain in accordance with signal strength of the received signal, and requires to operate with low distortion and low noise characteristics.
Presently, the DAB receiver for an auto-mobile use is appeared on the market, and in order to satisfy above-mentioned requirements, the high frequency signal amplifying circuit 2 of the DAB receiver employs a construction including an attenuation circuit such as a PIN diode at the front-end section.
FIG. 4 shows one example of a gain-controllable high frequency signal amplifying circuit used in the conventional DAB receiver. In FIG. 4, an input signal source 12 supplies the received signal in the above-mentioned example in FIG. 3, and the received signal from this input signal source 12 is amplified by an npn transistor 14 after passing through a PIN diode 13. In this case, the input signal source 12 may include a pre-stage circuit instead of the antenna 1. The signal from the npn transistor 14 is then obtained through a load inductor 15 and a load capacitor 16 at an output end 17 as an amplified signal.
A current source 18 is used for supplying a base bias current to the npn transistor 14. Further, a voltage of a DC (Direct Current) voltage source 19 is 3 volts in this example, and resultantly, 5mA of the base bias current is supplied to the a base electrode of the npn transistor 14 from the current source 18.
A current source 20 is an AGC current source in which current value is controlled by a gain control signal from the AGC circuit 11 in FIG. 3. In case of this example, the current source 20 flows current of about 10mA when the level of the received signal is below a predetermined level. In this condition, an On resistance value of the PIN diode 13 is low or almost zero, thereby the received signal from the input signal source 12 is supplied to the npn transistor 14 without attenuation.
As the level of the input signal increases while exceeding the predetermined level, the current value of the current source 20 is reduced by the gain control signal from the AGC circuit 11. Then the resistance value of the PIN diode 13 goes high, and thereby, the received signal supplied from the PIN diode 13 to the npn transistor 14 is attenuated at this PIN diode 13 and the gain of the high frequency signal amplifying circuit in FIG. 4 is lowered. Thus, the high frequency signal amplifying circuit in FIG. 4 is gain-controlled to make the level of the amplified signal to be supplied to the mixing circuit 3 to be a proper value in accordance with the gain control signal from the AGC circuit 11.
But the high frequency signal amplifying circuit in FIG. 4 for the auto mobile DAB receiver is not suitable for a portable radio receiver because of following problems. Namely,
1. The current consumption is 10 mA for the PIN diode and 5 mA for the npn transistor 14, and totally it becomes 15 mA. This amount of the current consumption is too large for a battery-operated portable radio receiver. PA1 2. A semiconductor fabricating process of the PIN diode 13 is different from that of the npn transistor 14, so that it is not suitable for a monolithic fabrication and not suitable for a large-scale integration.